Typically, bond-per-pixel-block stacked-wafer image sensor designs have a pixel array die with pixels arranged in groups or pixel blocks, with each group containing 8, 16, 32, or 64 pixels, each pixel of the group being read in sequence through an analog-to-digital converter (ADC) into an image memory (image RAM), at least the image RAM being located on a die different than the pixel array die.
Image sensors may generate huge amounts of raw image data. For example, a 24-megapixel image sensor having 12-bit ADCs may generate 288 megabits of data for each raw image frame. Some cameras capture not just a single image, but a burst of four, and sometimes more, images; four 288-megabit images exceeds a gigabit of memory required to store a single burst of raw images. Dynamic memory (DRAM) is preferred for image RAM because of its small cell size for storing such image bursts until images can be processed and compressed.